The present invention relates to a method for shielding from debris, a nuclear fuel assembly including a fuel bundle and a upper tie plate for supporting the fuel assembly and, particularly, relates to a debris shield that is releasable, for the purpose of cleaning or replacement, from an Upper Tie Plate (UTP) of the fuel assembly.
In a nuclear fuel assembly, liquid coolant/moderator flows into the assembly thru the bottom and exits as a water and steam mixture from the top during normal operation. The core of a nuclear fuel reactor includes a plurality of fuel assemblies arranged in vertical side-by-side relation, each containing a bundle of fuel rods. The fuel assemblies include a fuel bundle and a housing formed by a hollow metal channel. The fuel bundle includes an array of parallel fuel rods, water rods and one or more tie plates, spacers, and finger springs, that support the rods in the bundle. Generally, a fuel bundle includes an upper tie plate near the top of the bundle, which is removable and a lower tie plate at the bottom of the bundle, which is conventionally a non-removable unit.
Debris may fall through a conventional upper tie-plate and become lodged within the fuel bundle where the debris may cause fuel rod fretting during normal reactor operating conditions. Fretting is potentially damaging to the fuel rods, resulting in what is typically known as a “leaker”.
Conventional efforts to address debris falling down into a fuel assembly typically focus on the prevention of debris from entering within the coolant itself and coolant flow passages, prior to coolant flow entering the fuel assemblies within the core. Conventional efforts typically involve administrative controls regarding the treatment of coolant flow passages and handling of fuel assemblies such that debris does not enter the passages of the fuel assemblies. These controls are designed to alleviate the sources of debris such that debris does not fall down into fuel assemblies. Nevertheless, there is a risk that debris will fall down into a fuel assembly, especially while the coolant flow stops and the reactor core is open or when service work is preformed on the core.
There is a long felt need for procedures and special devices to ensure that debris does not fall into fuel assemblies from above, especially during refuel operations, fuel inspections and when the coolant is in a reverse coolant flow pattern. Further there is a long felt need for efficient and non-intrusive methods to insert, remove and clean devices associated with the UTP that capture debris that would otherwise flow down into a fuel bundle assembly from above.